Oil reservoirs are geological units within the subsurface of the Earth that contain an accumulation of oil. The oil from the reservoirs is extracted or recovered for use by a process commonly referred to as oil production. Conventional oil production typically involves two stages: primary recovery and secondary recovery. Primary recovery involves the use of natural in-reservoir high pressure forces to drive the flow of oil to oil production wells. Secondary recovery typically involves the maintenance of this high pressure by pumping fluids into the reservoir so that oil production may continue.
In oil reservoirs that contain heavy oil or oil-sands (also known as tar sands, bitumen, or bituminous sands), the oil is too viscous to flow freely to the production wells by conventional methods. As such, other means of oil production, such as thermal recovery strategies, must be employed. Thermal recovery strategies involve heating the oil reservoirs to improve the mobility of the oil and thus the ease of its subsequent extraction. The applied heat reduces the oil's viscosity allowing it to flow to production wells. An example of a commonly used thermal recovery strategy in heavy oil recovery is the Steam-Assisted Gravity Drainage (SAGD) method.
The SAGD method involves the use of steam injection well 5 and production well 7 pairs, as depicted in FIGS. 1, 2 and 3 of the prior art. The steam injection well introduces steam into a clean sand area 8 of an oil reservoir. The injected steam migrates upwards until reaching geological units that prevent further migration of the steam. The injected steam heats the reservoir to temperatures of approximately 200° C., reducing the viscosity of the oil and allowing it to flow to the oil production wells. This is referred to as the steam chamber 10, shown in FIGS. 2 AND 3. The steam is constantly injected to decrease oil viscosity, facilitating the continuous flow of oil towards the production wells, and to help displace the oil from the sand.
There are several drawbacks to the SAGD process. One major issue is that the SAGD steam-generation process has a negative environmental impact. For example, the SAGD process is a large contributor of greenhouse gas emissions. This is because large amounts of natural gas must be combusted to provide the energy to heat water to create the steam. Not only does burning natural gas contribute significantly to greenhouse gas emissions, it also represents an added cost for bitumen production. Furthermore, the SAGD process also consumes large amounts of water resources for the creation of the steam.
As the production of steam is the primary contributor to the environmental and economic impact of the SAGD method, the environmental efficiency of SAGD operations may be expressed in terms of the steam-to-oil ratio (Gates & Larter, 2014, incorporated herein by reference). The steam-to-oil ratio encompasses both the environmental and economic cost of steam generation in relation to the amount of crude oil resource that is recovered. A lower steam-to-oil ratio means fewer greenhouse gas emissions and improved environmental performance per unit of production.
The energy costs and greenhouse gas emissions associated with unconventional oil sands extraction and production, such as SAGD operations, are approximately 100-200% greater than for conventional oil production (“The Truth About Dirty Oil: Is CCS the Answer?”, Bergerson & Keith, Environmental Science & Technology, 2010, 44, 6010-6015, incorporated herein by reference). As such, new strategies and technologies for improving the environmental and economic performance in oil sands extraction must be developed to lower the steam-to-oil ratio associated with SAGD operations.
Production from conventional oil reservoirs is typically inefficient at extracting all of the available oil from the targeted region. As such, there are many strategies that aim to increase oil recovery. Some of these strategies include the use of microorganisms in the subsurface.
Subsurface environments are microbial habitats and include a wide variety of microbial taxa. FIG. 4 of the prior art shows a histogram of the abundance rank order of different microbial taxa in a subsurface environmental sample; adapted from Pedrós-Alió (2006) “Marine microbial diversity, can it be determined?”, Trends in Microbiology, Vol 14, No 6, pp 257-263. The bars of the histogram are indistinguishable as they are very close together. The lighter shaded area 1 on the left of the histogram represents abundant taxa, and the darker shaded area 2 to the right represents the rare taxa. Therefore, in a given environmental sample, there is often a large proportion of abundant and active microorganisms along with a variety of low abundance, inactive and/or dormant microorganisms. For example, in some microbial communities, one species might encompass up to 20% of the total cells present, whereas hundreds of rare species may collectively make up less than 1% of the total.
Microbially Enhanced Oil Recovery (MEOR) is a term to describe strategies for conventional oil production that target the use of microbial communities for enhancing and increasing oil recovery from conventional oil reservoirs. MEOR is typically employed after primary and secondary recovery. With MEOR, microbes are utilized in the conventional target regions of the reservoir to improve oil production. MEOR is believed to occur by a variety of mechanisms related to microbial metabolism in oil reservoirs, including biosurfactant production, metabolism of oil, and production of gas as a metabolic by-product. Each of the processes mentioned above helps to increase the fluid mobility of the oil, leading to the production of the residual oil still present in the reservoir after primary and secondary recovery strategies.
MEOR is typically attempted as a tertiary recovery strategy in conventional oil reservoirs. However, due to the unconventional nature of heavy oil and oil sands and the unconventional production methods for producing this oil, MEOR strategies are not frequently applied in heavy oil and oil sands.
MEOR may be applied to the commonly-targeted region of a heavy oil or oil sands unit before or after the application of strategies such as the SAGD method. MEOR involves either (1) biostimulation, i.e., the injection of nutrients to stimulate the native predominant and abundant taxa, or (2) bioaugmentation, i.e., the injection of foreign bacteria that are thought to be suitable for the reservoir conditions.
The high temperature of the SAGD steam chamber sterilizes the conventional target region of the oil sands reservoir. Therefore, when MEOR is utilized for enhancing oil recovery from the SAGD steam chamber of a heavy oil sands reservoir, MEOR may only be applied either before the steam is injected into the reservoir, or after the SAGD method is complete and the reservoir has cooled down to low temperatures. U.S. patent application Ser. No. 14/070,095, incorporated herein by reference, describes a method of injecting foreign bacteria prior to injecting steam as a part of SAGD for increasing the fluid mobility of oil in a heavy oil reservoir. In this method, microorganisms are introduced into the reservoir through both injection and production wells, prior to steam injection, to pre-condition the reservoir for enhanced (shorter) start-up of the SAGD process.
U.S. Pat. No. 4,475,590, incorporated herein by reference, provides an example of biostimulation in a conventional oil reservoir in conjunction with waterflood technology. Waterflooding aims at displacing the residual oil in the reservoir with water, rather than the steam that is applied during the SAGD method. Similarly, U.S. Pat. Nos. 4,971,151, and 5,083,611, incorporated herein by reference, describe methods involving the injection of nutrients in the conventional oil reservoirs for enhancing oil recovery.
All of these methods, however, focus on the active taxa present in high relative abundance in the microbial communities that are adapted to local prevailing in situ conditions (temperature, geochemistry, salinity, mineralogy, etc.) and that are readily investigated by microbiological methods. Yet, in nearly every environment there are microbial seed banks that include many species or taxa of microorganisms present in low relative abundance. These microbial taxa can be inactive or dormant, and may include dormant bacterial endospores. Microbial seed banks may constitute significantly less than 0.01% of the total cells present, and often exist in a dormant state. As such, they are typically not detected or highlighted by most environmental DNA extraction surveys, and other more traditional methods for microbial characterization of oil reservoir environments.
Furthermore, the subsurface regions beyond the boundaries of the SAGD steam chamber, such as inclined heterolithic strata (IHS), may contain up to twice as much oil sands resource as the targeted steam chamber region. However, production of the oil in the IHS region during SAGD is limited. This IHS oil is interbedded with thin, but laterally extensive, low-permeability mudstone layers through which the steam cannot penetrate. Therefore, methods other than gravity drainage are required to displace the oil. The oil in the IHS is considered higher quality and more valuable than the oil in the steam chamber region as it is less biodegraded and less viscous (“Impact of oil-water contacts, reservoir (dis)continuity, and reservoir characteristics on spatial distribution of water, gas, and high-water” Fustic et al., 2013, Heavy Oil/Bitumen Petroleum Systems in Alberta & Beyond, Eds. F. J. Hein, J. Sutter, D. A. Leckie, and S. Larter, AAPG Memoir, p. 163-205., incorporated herein by reference in its entirety).
FIG. 5 shows a schematic of an example of a commonly-targeted geological unit in the subsurface of the Athabasca oil sands. The lower region represents the target for steam chamber 10 placement, which is the targeted region for SAGD. The upper region represents the IHS region 20, which contains oil that is not easily accessible by current methods. Limited oil recovery is documented from the IHS. The diagonal lines in the IHS region represent the laterally extensive mud strata 30 interbedded with decimeter scale heavy oil or bitumen saturated laterally extensive porous sands. Above and below these regions are the low-permeability non-reservoir underseal 22 and seal 25.
FIG. 6 shows a photograph of an Athabasca Oil Sands outcrop near Fort McMurray in Alberta, Canada by Strobl et al. (1997) from the Canadian Society of Petroleum Geologists, Memoir 18, pp 375-391. The geological unit shown in FIG. 5 is representative of the geological unit in the Athabasca Oil Sands. Referring back to FIG. 6, the white substantially parallel lines along the upper half of the geological unit represent the laterally extensive mud strata 30 of the IHS region 20, and have a slope of approximately six (6) to ten (10) degrees. The lowest laterally extensive mudstone layer 35, as denoted by the arrow, defines the expected upper boundary of the SAGD steam chamber 10 (as demonstrated by subsurface studies, Strobl et al., 1997, Strobl, 2013).
While means to increase oil recovery from the accessible regions, such as the SAGD steam chamber, are widely researched, access to the oil in the IHS layer remains challenging with use of existing technologies. There are many initiatives to try to access this oil, such as by attempting to break the mudstone in the IHS by geomechanical, electrical, Enhanced Solvent Extraction Incorporating Electromagnetic Heating (ESEIEH), or thermo-chemical processes to access the oil. However, these approaches have had a very limited success thus far.
There is therefore a need to mitigate, if not overcome, the shortcomings of the prior art and to, preferably, develop a method to produce oil or increase oil production from currently challenging IHS regions of oil reservoirs.